THE EVOLUTIONARY CAREER 305 



the carbon assimilation habit became prevalent. Thus the 

 atmosphere would tend to its present composition. 



And as the volume of the ocean increased the processes of 

 precipitation of water and snow would also increase. The bare 

 surface of igneous rock would become eroded and weathered and 

 true soils would appear. Then land faunas and floras would 

 appear. 



All this preliminary, but most significant, evolutionary process 

 must have gone on during Archeozoic and Proterozoic times. 

 Apart from those of Algae and Sponges there would be no organic 

 skeletons of mineral composition and therefore susceptible of 

 fossilization. In all this vast period of time (nearly half of the 

 whole life-period) hardly any permanent remains of animals and 

 plants were formed and such as may have been fossilized are now 

 irremediably destroyed by the extensive alterations of the Pre- 

 Cambrian sediments. We have therefore no records that indicate 

 what were the structures of the earliest living things and we 

 can only doubtfully infer what may have been their modes of 

 metabolism. It is certain that, at the beginnings of the Paleozoic 

 period all the great types of animal and plant organisms (except the 

 land plants) had been evolved. It is probable that these great 

 types did not evolve from a single organic stock but from several 

 such stocks, as is indicated by the various modes of metabolism 

 that were possible ones. 



We can therefore only attempt to describe the evolutionary 

 career as it is laid open to us in the fossil remains that are disclosed 

 in the Paleozoic and upper sedimentary rocks. 



102. ON ''LINES OF DESCENT'' 

 We think of some natural region as being populated by animals 

 of a particular kind, or species, all freely interbreeding and 

 reproducing (as is generally the case, in the wild,) once a year. 

 There are perhaps millions of individuals in the population and 

 once a year a certain fraction of these reproduce so that there is 

 a new " generation " of the species at about the same season in 

 every year. On the average the population is constant so that 

 as many individuals must die as are born during any specified 

 period. Such a stock may continue to populate the region, 

 remain constant in numbers and exhibit no change in characters, 



